1. Field of the Invention
The present invention relates to a method of tracing a conductor e.g. the route of pipes and cables; and to an apparatus therefor. By cable we include electric cables, optical fibre cables (where the conductive armouring provides a conductive path). Indeed the invention is applicable to any metallic pipe or duct. It is now well-known to induce a signal into a metallic conductor, and to detect the propagation of that signal at a remote point, to detect the conductor's position or faults in the conductor. This technique is particularly, but not exclusively, applicable where the conductor (such as an electric cable) is inaccessible, or hidden e.g. underground.
2. Summary of the Prior Art
Many techniques have been developed for analysing the signals which propagate down the conductor, and various techniques have been established for remotely detecting those signals. In general, the field produced by the alternating current on the service may be sampled using an aerial placed in the vicinity of the conductor; the relative magnitude of the detected signal at various points conveys information about the conductor's position. One particular configuration uses one or more horizontal coils so that the detected signal is a maximum when the coil is directly above and orthogonal to the conductor. The phase of this signal with respect of the transmitted signal will be constant at a point, the value determined by the electrical properties of the circuit.
However, much of the theory of such detection has been determined on the basis that there is a single conductor. In practice, such conductors are usually in groups. The problem then is that, although it is possible to ensure that the transmitter induces current only into one conductor, nevertheless there is likely to be capacitative coupling or even direct bonding between the conductors along their length, and therefore currents will also be generated in the other conductors. At first sight, these other currents should be of much lower amplitude, but in fact the detected signal, derived from the magnetic flux seen by the receiving aerial may not differ significantly between one conductor and another. Bearing in mind that the detection of the signal may have to be from a point remote from the conductor, the distance between the detector and the conductor may therefore be a significant factor in the magnitude of the signal detected. For example, if the conductor to which the current is actually applied is somewhat further away from the detector than another conductor to which current has capacitively been transmitted, then the signal from the latter conductor may be of comparable magnitude with that in the conductor to which the current has been applied. As a result, it is impossible to tell the two apart, and thus the measurement is ineffective.